Guide vanes are known to comprise blades that have one end with an outer platform connected to a guide vane carrier or gas turbine casing, and an opposite end provided with an inner platform facing the rotor.
Moreover, in order to withstand the high temperature of the hot gases flowing in the hot gases path, guide vanes (and in particular those closest to the combustion chamber) have an inner cooling circuit.
This inner cooling circuit comprises a path that makes a cooling fluid (typically compressed air) to circulate within the guide vanes and be ejected into the hot gases path through cooling holes provided over the guide vanes.
For manufacturing reasons (otherwise it would not be possible for example to realise the cooling circuit), the guide vanes have apertures at both the outer and inner platform; the inner platform is then closed by a cover plate.
Traditionally, the inner platform has a housing with L-shaped sides, such that the cover plate is inserted in the housing with the terminal portions of the L-shaped sides that withhold it; then the cover plate is also connected (usually brazed) to the platform.
Periodically the cover plates must be removed from the housing to be reconditioned; after the cover plates are removed, new or the same cover plates (after reconditioning) must be connected in the housing of the inner platform.
Nevertheless, removal of the cover plates is carried out by milling the L-shaped sides of the housing.
Thus, when the cover plates are re-inserted into the housing, they are only brazed thereto, but there is no mechanical withholding due to the L-shaped sides of the housing.
The brazing connection is not as reliable as the mechanical connection and in some cases could break.